Effects of progressive changes in organoalkoxysilane structure on the gelation and pore structure of templated and non-templated sol-gel materials

We study how progressive changes in silane structure affect the synthesis and properties of organosilicas. Tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), methyltrimethoxysilane (MTMS), bis(trimethoxysilyl)ethane (BTMSE), bis(trimethoxysilyl)hexane (BTMSH), and bis(trimethoxysilylpropyl)amine (BTMSPA) are used as precursors in non-templated base and acid-catalyzed sol-gel processes, and in templated processes with cetyltrimethylammonium bromide (CTAB) and polyoxyethylene 10 lauryl ether (C₁₂E₁₀). The gel time of materials made without templates is mainly controlled by the structure of the silane rather than its reactivity. For instance, a dangling methyl group (MTMS) inhibits gelation, while a short bridging chain (BTMSE) promotes gelation. In basic conditions, mesoporous materials are obtained with TEOS and TMOS, while microporous materials are obtained with organically modified silanes without added amine. Dipropylamine, originally added as a catalyst, in fact templates mesopores in BTMSH-derived organosilica. In acidic conditions without pore templates, all products are microporous. In the presence of CTAB, mesopore templating occurs with TEOS, TMOS, BTMSE, and BTMSPA. With C₁₂E₁₀, mesopore templating occurs with TEOS, TMOS, and BTMSE. Surprisingly, the BTMSE-based material has the most uniform mesopores of all samples in the C₁₂E₁₀ series. Mesopore templating fails when a dangling organic limits the formation of stable pore walls (MTMS) or a large hydrophobic chain disrupts the formation surfactant micelles (BTMSH).